Despite the fact that the heat pump was first conceived by Lord Kelvin more than a century ago, operating difficulties and economic considerations have severly restricted its use in practical applications.
In its most simple form the heat pump (the term commonly employed when heat is the desired product) or refrigeration cycle (when cooling is desired) is a simple closed system containing a refrigerant such as ammonia or a volatile fluorocarbon. The closed system is usually comprised of an evaporator, a condenser, a compressor, an expansion valve, and various controls. In operation the liquid refrigerant evaporates at a low temperature and pressure inside the evaporator usually a tubular metal coil) and extracts from the surrounding area a quantity of heat equal to its heat of vaporization. The vaporized refrigerant flows to the compressor where it is compressed to a pressure and temperature sufficiently high so that it will condense in the condenser (usually a tubular metal coil), thus giving up its heat of vaporization. In a refrigeration cycle, the heat released in the condenser is wasted to a stream of cooling water or to the atmosphere. In a heat pump the heat released in the condenser is most commonly used to heat a building but is sometimes put to other use. The ASHRAE 1972 Guide and Data Book, Chapter 43, describes the components and operation of a typical heat pump cycle.
The principal obstacle to widespread use of the heat pump cycle has been a lack of a satisfactory heat source. Most heat pumps, up until the present time, have employed ambient air as a heat source due to its universal availability. Unfortunately, such air based systems become uneconomical if the available air temperature drops below about 40.degree. F. It will be appreciated that when heat is extracted from air, the air is cooled and as a result it becomes necessary to move large volumes of air to avoid too great a temperature drop. This requires a very large fan which may be noisy and which consumes a disproportionately large amount of energy. A further temperature drop occurs through the metal heat-transfer surface of the evaporator and as a result, when the ambient air temperature is 40.degree. F, the vaporizing temperature of the refrigerant may be only 10.degree. F. or less. At ambient air temperatures below 40.degree. F. these problems become more severe and operation becomes uneconomical requiring the use of supplemental heat sources. This problem is further aggravated by the build-up of an insulating layer of frost on the metal surfaces due to the freezing of moisture contained in the air. This build-up of frost seriously impairs efficiency and must be removed periodically.
Other heat pump systems use heat sources other than air, such as well water from lakes, streams and oceans. Still others use solar energy or heat from the earth. All of these sources have serious disadvantages such as scale build-up, when ocean water or well water containing minerals is used. Lake and stream water does not provide sufficient heat in winter months when it is most needed. Solar systems and earth systems are very costly to install and solar systems suffer the further disadvantage of unreliability.
It is therefore the principal object of this invention to provide a heat pump system having as an integral element, a heat source which is universally available and which is capable of providing heat at a fixed temperature regardless of ambient temperature and atmospheric conditions.
A further object of this invention is to provide a heat pump system which is not subject to corrosion, scale or ice build-up and which can be installed and operated at relatively low cost.
In accordance with this invention these and other objects are achieved through the use of the heat liberated by the heat of crystallization of water as it freezes. This heat amounts to 144 BTU/lb. of ice which in the case of pure water, is released at 32.degree. F. The usual problems of icing, scaling, and corrosion of the evaporator (the heat-absorbing unit), are avoided by carrying out the freezing in a separate vessel by boiling the water under vacuum at its triple point (about 4.6 mm Hg absolute pressure). The heat released by the crystallization of the water is absorbed as the heat of vaporization and forms an equivalent quantity of water vapor which by compression and condensation can be made to liberate its heat of condensation for useful heating. All heat transfer in such a triple-point freezer occurs at the water surface. As a result there is no metal surface on which to collect ice, form scale, or to corrode. The heat liberated on condensation is approximately equal to the heat of crystallization liberated by the formation of ice, plus the energy consumed in compressing the water vapor. Both the condensed vapor and the ice are pure water, regardless of impurities which may be present in the water being frozen.
The pure water produced by this process may be discarded or used as a source of potable water. The ice produced can be used for cooling or may be melted and also used as a source of potable water.
The heat content of the compressed vapor can be used in a variety of ways as hereinafter described.